Question

Define electric dipole moment. Give its unit.

Answer 1

Hint: When two equal and opposite charges are separated by a distance, there will be some attraction between the charges. It is either in an atom or in a molecule. In some molecules, the positive and negative charges lie on the same plane whereas in some other they lie on different planes. This leads to attraction between the charges in the atom.

Step by step solution:
(i) Electric dipole:

The electric dipole is a pair of two equal and opposite charges, $ + q$ and $ - q$ are separated by a very small distance. The algebraic sum of the charge on the dipole is zero and the electric field of the dipole is not zero, thus the electric field of dipole is added.
Examples of electric dipole are $C{H_3}COOH$, $HCl$, ${H_2}O$ etc…
These molecules have permanent electric dipole moment because the centre of positive charge is not coinciding with the centre of negative charge in these molecules.

The figure shown above is the dipole of ${H_2}O$, the $P$ shown in the figure is the positive charge side and it is not coinciding with the centre of negative side.

(ii) Electric dipole moment:

The electric dipole moment of the electric dipole is defined as the product of the magnitude of one of the charges of the dipole and the separation distance between them. The figure shown below is the electric dipole.

From the above figure, it is clear that $d = 2s$
Electric dipole moment = one of the charge value * separation vector from -ive charge to +ive
$\mathop p\limits^ \to = q \times 2\mathop s\limits^ \to $
Where, $\mathop p\limits^ \to $ is the electric dipole moment, $q$ is the magnitude of one of the charges and $\mathop d\limits^ \to $ is the separation distance.
Since the dipole moment and the separation distance is the vector quantity. So, they are denoted as $\mathop p\limits^ \to $ and $\mathop d\limits^ \to $.
Hence,
$\left| {\mathop p\limits^ \to } \right| = q \times 2s$

The electric dipole moment is always directed from negative charge to positive charge.
The S.I unit of electric dipole moment is Coulomb-meter $\left( {Cm} \right)$

(iii) Significance of electric dipole and electric dipole moment:
1. Matter which is made up of atoms and molecules is electrically neutral. If the centre of mass of positive charges coincides with that of negative charges then the molecule behaves as a non-polar molecule. On the other hand, if the centre of mass of positive charges does not coincide with that of negative charges then the molecule behaves as a polar molecule. These polar molecules have permanent dipole moments. These dipole moments are randomly oriented in the absence of an external electric field. If we place a material with polar molecules in an external electric field then these molecules align themselves in the direction of the field. This results in the development of a net dipole moment. This particular piece of material is said to be polarized.
2. Atoms as a whole are electrically neutral in their ground state. We know that atoms have equal amounts of positive and negative charge. Similar to atoms, molecules are also neutral but they also have equal amounts of positive and negative charges.
3. Presently when in a system, the logarithmic total of the considerable number of charges is zero it doesn't really imply that the electric field created by the zero is zero all over. This makes study of electric dipoles significant for electrical phenomenon in matter.

Note: There is another term called ideal electric dipole. The ideal state in an electric dipole when the charge gets larger and the distance of separation gets smaller. In that case, the dipole moment $p = 2 \times 2s$ has the finite value. Such a dipole with negligible small size is called ideal or point dipole.